In recent years, depletion of fossil fuel such as petroleum and coal is concerned, and expectations for sustainable renewable energy are increasing. Examples of the renewable energy include solar cells, wind power generation, and the like. The amount of power generated by them depends on weather and nature conditions, and thus they have a problem of difficulty in stable supply of power. Accordingly, there are attempts to store electric power generated by the renewable energy in a storage battery, so as to stabilize the electric power. However, when the power is to be stored, there are problems such as requiring costs for storage batteries, and occurrence of loss while storing the power.
For such points, techniques are gaining attention to use power generated by renewable energy to electrolyze water, so as to produce hydrogen (H2) from water, or electrochemically reduce carbon dioxide (CO2) to convert it into chemical substances (chemical energy) such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), methane (CH4), ethanol (C2H5OH), ethane (C2H6), ethylene (C2H4), and the like. When these chemical substances are stored in a cylinder or a tank, there are advantages that storage costs of energy can be reduced as compared to the case where the power (electric energy) is stored in a storage battery, and that storage loss is small.
A typical electrolytic device for carbon dioxide is a device having an electrolytic bath accommodating an electrolytic solution containing water (H2O) and carbon dioxide (CO2), and an anode (oxidation electrode) and a cathode (reduction electrode) which are immersed in an electrolytic solution. When hydrocarbon or the like is produced by an electrolytic reaction of carbon dioxide, a metal electrode of copper, copper alloy, or the like is used for the cathode. When the conventional electrolytic device is used, for substances having a small number of electrons used for a reduction reaction of carbon dioxide, such as carbon monoxide or formic acid, a good amount of production is obtained. On the other hand, for substances having a large number of electrons used for a reduction reaction of carbon dioxide, such as ethanol, ethane, ethylene, methanol, or methane, the amount of production is small in the current situation. The substances such as ethanol, ethylene, ethane, methanol, and methane are of high usefulness compared to carbon monoxide, and thus there are demands for increasing the amount of production of these substances as a reduction product of carbon dioxide.